Excited-state dynamics

The spectral dynamics of DCM after photoexcitation from the electronic ground state are characterized by strong excitation intensity dependence for all excitation wavelengths and solvents investigated. This is manifested in the pump-probe spectra, especially in the time evolution of their total spectral intensity. With increasing excitation energy the amplitude fraction of a non-instantaneous contribution to the decay of the total spectral intensity increases (Figure 4.1-22). This decay is ascribed to the rise and decay of stimulated emission in the ranges above and below approx. 575 nm in acetonitrile and methanol or approx. 525 nm in chloroform, toluene, tetrachloromethane and cyclohexane (4.1.2.2.). The spectral dynamics could be described by precursor-successor kinetics (4.1.5.), the analysis yielding a time coefficient τ of 0.23 (_± 0.04) ps averaged over the dynamics in all solvents investigated. This initial relaxation is followed by a red shift and further rise of the emission band in acetonitrile and in methanol, whereas in chloroform and in toluene the continued rise of the emission above 525 nm is accompanied by a further decay of emission below this wavelength.

The excitation energy dependence cannot be explained by saturation; it is interpreted as due to resonant two-photon absorption of DCM. Feeding of the S₁ from higher electronic states has been proposed before for DCM [Kov 96, Ruth]. Very high effective non-resonant two-photon absorption coefficients have been reported for bis-donor substituted stilbenes in

acetone and toluene by Ehrlich et al. [Ehr 97]. From a comparison of measurements with the same excitation energy, but different pump wavelengths of 530 and 470 nm in Figures 4.1-1 c) and 4.1-10 b), the larger relative amplitude of the blue shifted emission in the data for 470 nm excitation indicates a wavelength selectivity of the two-photon absorption process.

The excited state absorption, as obtained from the spectral decomposition (fig. 4.1-1 b), has its maximum at 465 nm in acetonitrile. The two-photon absorption process may therefore be pictured to be resonant, with the first singlet excited state as an intermediate level. The transition strength for the excited state absorption can be roughly estimated from the spectral decomposition to about 1.4 x the transition strength for S₀→S₁ absorption. The maximum extinction coefficient for ground state absorption has been determined (cf. 4.2.1) as ≈ 43300 l mol^-1 cm^-1 in acetonitrile, so that the maximum exctinction coefficient for the excited state absorption is ≈ 61500 l mol^-1 cm^-1. The absorption cross section σ can be calculated from the extinction coefficient after σ = 1000 ln(10)ε⋅ N_Ato maximum values of σ = 1.66 x 10^-16 cm² for the S₀→S₁ transition and σ = 2.36 x 10^-16 cm² for the S₁→S_N transition.

The transition moment d is related to the absorption spectrum according to [Lip 68] by:

d N d From the intensity of a lognormal approximation to the ground and excited state absorption bands, the transition moments d₀₁ and d_1N were calculated after equation 5.1 to 2.95 x 10^-29 and 3.02 x 10^-29 Asm, respectively.

A cross section of two-photon absorption ~σ may be defined which is normalized by the photon flux per unit area I = P /Aω:

σ~ = / I . (5.2)σ Schubert and Wilhelmi [Schub] deduced the following expression for ~σ :

~ ( ) and g(ω−ω0) is the absorption line shape function. For simplicity, a Lorentzian lineshape is assumed, with

g( ) /

α is the polarizability matrix element for the transition between levels 0 and N. If the intermediate level 1 between the ground state and the state N reached by two-photon absorption is far from resonance, the polarizability can be approximated by [Schub] :

α⁰ ω achieve effective non-resonant two-photon absorption. The 50 fs, 470 nm-centered pulse with a diameter of 150 µm on the sample and 1 µJ pulse energy such as employed in the pump-probe experiments corresponds to an intensity of 1.1x 10¹⁵ J/m²s and a photon flux of

≈ 2.7 x 10 ³³ m^-2s^-1 . This yields ≈ 0.54 x 10 ^-16 cm² for the two photon cross section and thus raises it to about one third of the cross section σ for the one-photon transition to the first singlet excited state. In the case of resonant two-photon absorption, the interaction cross section may increase by up to 10⁷-10⁸ [Letok]. Unfortunately, the excitation energy could only be varied in a range from 0.2−1.4 µJ, the lower limit determined by the signal-to-noise ratio and the higher limit imposed by saturation of the S₀→S₁ transition. Therefore, and because of the overlap of the excited state absorption band, a more thorough study of the excitation intensity dependence of the blue shifted emission could not be carried out.

The shift of the early emission bands in the pump-probe spectra of DCM in chloroform when changing from a 470 nm excitation wavelength to 530 nm excitation (figures 4.1-12 b) and 4.1-13), leads to the assumption that at least in chloroform, several higher-lying electronic states may be reached by two-photon absorption. This is roughly confirmed by the semiempirical calculations yielding three transitions between 38000 and 41000 cm^-1.

In the limit of zero excitation energy, the time coefficient τ for the decay of the integrated spectral intensity reduces to approx. 0.09 ps in acetonitrile (4.1.4.), close to the timescale of inertial motion for acetonitrile molecules of ≈ 100 fs [Cho 92, Horng 95, Ruth 98]. A

similar tendency towards lower values for τ in chloroform and tetrachloromethane cannot be relied on, because the large scattering of values from different days of experiment would require a larger data set. The trend in acetonitrile can be interpreted in two ways: either a very fast reaction takes place or is observed only for low excitation energy, or solvent relaxation gradually alters the transition strength for S₁→S₀ fluorescence. The lack of excitation energy dependence of the time coefficients from the precursor-successor fits and the fact that there is no fast decay of the emission in the blue and green spectral region support the second assumption. It may also explain the absence of a decrease of τ with excitation energy for methanol, where most of the energy relaxation from solvent reorientation is slower than 0.2 ps (see discussion of LDS-750 results). The solvent-dependent continued rise of the emission band in acetonitrile and methanol on a picosecond timescale would also be in line with a transition strength dependence on solvent orientation.

It could not be examined whether these amplitude changes are reflected in the total intensity on a picosecond timescale, because the noise level is too high to detect a small decay component of the integrated spectral intensity. The amplitude rise could also be due to emission band narrowing as observed by Glasbeek et al. and Gustavsson et al. [vdMeul 98, Gust 95].

On the other hand, the appearance of a second band around 460 nm in addition to that at 540 nm (fig 4.1-10) with higher excitation energy indicates a three-state scheme such as proposed by Ruthmann [Ruth], who found evidence for a charge transfer reaction of DCM in S₁ in methanol with τ = 246 fs and feeding of the reactant state from an emissive higher-lying electronic state with τ = 120 fs.

Figure 5.1-1: Three-state scheme for DCM relaxation in methanol as proposed by Ruthmann.

This cannot be corroborated here, as no indication of a decay as fast as 120 fs resulted from the precursor-successor modelling at high excitation energies. If there are three states involved in the relaxation, the reaction rate appears not to be influenced by the reaction path. It is also nearly independent of the solvent (table 4.1). The absence of dielectric relaxation effects on electron transfer have been reported for porphyrine-dichloroquinone cyclophanes [Pöll 92], so that a charge transfer reaction from two precursor states, the population of which depends on excitation energy, might be thought of.